1. Field of the Invention
This invention relates to semiconductor devices and more particularly to a switch for electro-optical signal processing.
2. Description of the Prior Art
In the prior art, the output current signal from a number of photodetectors were each coupled to one of two summing buses by utilizing MOS switching transistors. The photogenerated signal current from the photodetector may have frequency components or a bandwidth as high as 1 GHz. The signal photocurrent may be typically in the range from 10.sup.-8 to 10.sup.-6 amps. The frequency response of the switching transistors is limited because the 3 db corner frequency, f.sub.c, of the transistor switches is governed by the input node capacitance, C, and the transconductance, g.sub.m, according to the relation f.sub.c =g.sub.m /2.pi.C.
At low current levels, for example at 10.sup.-6 amps, the g.sub.m of MOS and/or bipolar transistor switches is limited by and directly proportional to the current flowing through the switch. The frequency response of the transistor switches may be increased by adding an auxiliary dc or bias current to obtain a higher g.sub.m. However, the additional current flowing through the transistor switch produces a significant additional noise current which further corrupts the photogenerated signal. The additional noise from the auxiliary bias current reduces the signal to noise ratio of the photogenerated signal after passing through a transistor switch.
The transistor switches referenced in this application are normally preprogrammed by a shift register. The switches remain in their preprogrammed state for long periods of time or can be changed electronically with the shift registers. The device is utilized when the transistor switches have realized their steady state condition.
It is therefore desirable to provide a programmable device for steering of low level high frequency, for example .apprxeq.1GHz, current between one or more summing buses beyond the transconductance limitation encountered in conventional MOS and/or bipolar switches.
It is further desirable to provide a structure for steering currents in the range from 10.sup.-8 to 10.sup.-6 amps having frequency components as high as 1GHz from a source of electronic charge, e.g. radiant energy absorbed in a photodetector, to one of two summing buses.